This invention relates to wireless data networks and more particularly to a multiple-hop wireless radio frequency mesh network routing scheme employing a packet switched frequency-hopping spread spectrum time-sharing communications protocol. This invention has particular application to data collection from an array of sensors disposed in a topology wherein at least two intelligent communication nodes are within reliable radio communication range within a matrix of peer communication nodes.
Wireless mesh networks employ intelligent nodes comprising a transmitter and receiver, a power source, input devices, sometimes output devices, and an intelligent controller, such as a programmable microprocessor controller with memory. In the past, wireless mesh networks, such as the internet, have been developed having configurations or networks for communication that are static, dynamic or a hybrid of static and dynamic. Power for these networks has been most often supplied via wires (the nodes are “plugged in”) or occasionally from batteries. As the size, power, and cost of the computation and communication requirements of these devices has decreased over time, battery powered wireless systems have become smaller and smaller. The limit to size scaling resulting from this trend to smaller and lower power wireless devices is in the millimeter size range, leading to predictions of “smart dust”. For this reason, the research community has adopted the name mote to refer to a small wireless sensor device. Mote is an old English word meaning a speck of dust.
A self-contained unit of communication information is called a packet. A packet has a header, a payload and an optional trailer (FIG. 2). A link is a path which originates at exactly one node and terminates at exactly one other node. A node is thus any vertex or intersection in a communication network. A node may be passive or intelligent. In the present invention, a mote is assumed to be an intelligent node in that it is capable of receiving and analyzing information, taking certain actions as a result of received information, including the storing of received or processed information, modifying at least part of received information, and in some instances originating and retransmitting information.
In ATM systems, a cell is a channel-specific time period of fixed duration during which a unit of communication occurs between two fixed terminals without conflict. By comparison, as used herein, a slot refers to a time period during which a packet can be sent as well as acknowledged, and a cell refers to a particular slot and radio channel offset in a superframe (defined below). In conventional TDMA systems, such as defined by the DS-1 (T-1) standard, a frame is a period of time of defined and fixed duration. By contrast, as used in connection with the present invention, a superframe is an arbitrary number of slots and thus can be of variable duration. A superframe is iterated each cycle, as hereinafter explained.
According to the invention, communication between intelligent nodes occurs only at specific times and on specific channels. Each intelligent node in a network represents its connectivity to other intelligent nodes in the network as a collection of directed links on one or more digraphs. Each superframe repeats in a continuous sequence of cycles. Each link can be used for the transmission and optional acknowledgement of a single packet. Thus, in a given superframe, the available bandwidth from intelligent node A to intelligent node B (in packets per second) is the product of the number of links from A to B in the superframe (links per cycle) and the superframe rate (cycles per second). For example, if there were 1 link from intelligent node A to intelligent node B in superframe S, and superframe S consisted of 100 slots of duration 50 ms per slot, then the length of a single cycle of superframe S would be five seconds (100×0.05=5), and the superframe rate would be 0.2 cycles/second. With one available link per frame, intelligent node A would be able to send at most one packet to intelligent node B every five seconds. In the same superframe, intelligent node B might have ten links to intelligent node A, giving B ten times the bandwidth to A as A has to B. In a separate superframe F with 10 slots of length 50 ms, intelligent node A might have five links to intelligent node C, giving an available bandwidth of 10 packets per second (1 packet/link*5 links/cycle*2 cycles/second).
The ability to create multiple superframes of different lengths, and assign different numbers of links between intelligent nodes in each superframe provides flexibility to the network designer. This flexibility allows bandwidth, redundancy, latency, and many other network performance parameters to be traded off against power consumption.
There is a one to one correspondence between digraphs or networks and superframes. Digraphs are the abstract representation of a superframe, and they allow designers to look at and design collections of links and understand their function. Each link in a digraph is assigned a cell, that is, a particular time slot offset and channel offset, in the corresponding superframe. In each cycle of the superframe, these two offsets are used together with the cycle number to calculate the exact time and frequency on which the intelligent node is to turn on its radio.
A circuit switched network is a communication network in which a fixed route is established and reserved for communication traffic between an origin and an ultimate destination. A packet-switched network is a communication network in which there is no reserved path between an origin and a destination such that self-contained units of communication traffic called packets may traverse a variety of different sets of links between the origin and the destination during the course of a message.
Packet switched networks, as opposed to circuit switched networks, are susceptible to multiple simultaneous communication and connectivity with competing data sources, so that packet collisions can result. Absent control over access, such networks suffer from inefficiency of bandwidth utilization and capacity limitations due to collisions among packets transmitted at the same time and sharing the same spectrum. Consequently, packets may be lost and thus must be retransmitted to complete message, resulting in substantial loss of efficiency.
The efficiency of communication is particularly critical in applications calling for extremely small size and low power consumption, where intelligent nodes are spaced relatively close to one another with the potential for substantial interference.
Frequency hopping spread spectrum systems are known, such as the systems derived from the technology of Metricom, Inc., now out of business, which developed and deployed the UtiliNet and Ricochet networks still in use. In such a system, nodes in communication with one another track a pre-agreed-upon pseudo-random frequency hopping pattern in order to maintain a communication link. Several systems could co-exist in a topology without undue interference.
By contrast, ATM adapts circuit switched systems to support packet communications. ATM stands for Asynchronous Transfer Mode and refers to a specific standard for a cell switching network with a bandwidth from 25 Mbps to 622 Mbps. In ATM systems, channel-specific time periods called cells are assigned to carry packets and route packets via these cells from an explicit source to an explicit destination in accordance with a circuit switched model. The speed of switching is enhanced by rapid examination of routing information in packet headers.
The virtual circuit in an ATM system is like a fluid pipeline: it starts in one place and ends in another and may zigzag as it goes through various pumping stations, but topologically it is a continuous straight line. The paradigm of the Internet is packet switched network. A packet switched network is analogous to an airline: in principle one could fly from coast to coast via various routes through any number of different cities, but booking with a particular airline results in a flight route through a particular node or hub city, such as Chicago. If you get to Chicago and the plane originally scheduled to fly to the ultimate destination, such as New York is out of service, it is normally necessary to re-book the remainder of the flight route via a different plane or intersecting airline service.
Also well known in the art are various packet based protocols, such as X.25 and TCP/IP, both of which typical employ in part source routing, namely explicit routing between source and destination in a packet switched model. These have been described in various readily available standards.
In order to further understand the background of the invention, it is helpful to understand a number of related concepts. Referring to FIG. 1A, a graph is defined a collection of vertices or intelligent nodes with connections, or links, between the intelligent nodes. Referring to FIG. 1B, a digraph is defined as a graph where all of the links have an associated direction, so that a digraph connects a plurality of intelligent nodes in a network with links defining direction of flow. Referring to FIG. 1C, a multi-digraph is defined as a digraph in which there exists at least one pair of links which both originate at the same originating intelligent node and terminate on the same terminating intelligent node. It is possible to have multiple multi-digraphs, if there is a first multi-digraph in which each link is labeled “1”, and a second multi-digraph in which each link is labeled “2”, and one or more of the intelligent nodes in the first graph is also in the second graph, then this is an example of multiple multi-digraphs.
Herein the concept of digraph-based packet transport is introduced. Digraph based packet transport is analogous to water flowing in a river delta with its meandering branches. If a number of intelligent entities each in an independent unpropelled watercraft were dropped all over the delta with no means of guidance except to choose a path at each fork, they would take a wide variety of paths, depending on flow and congestion. Eventually, all would arrive at the basin. Two that started far apart might end up close together, and two that started near each other might take completely different paths and arrive at different times.
A number of patents and publications provide background on other approaches to packet communication. Examples of instructive patents include: U.S. Pat. Nos. 4,550,397; 4,947,388; 4,939,726; 5,007,052; 5,079,768; 5,115,433; 5,130,987; 5,471,469; 5,488,608; 5,515,369; 5,570,084; 5,903,566; 6,735,178.
Instructive Publications:
IEEE 802.15.4-2003 IEEE Standard for Information Technology-Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low Rate Wireless Personal Area Networks (LR-WPANs) (Also available as online subscription at http://standards.ieee.org/catalog/oils/lanman.html).
Hohlt, Doherty, Brewer, “Flexible Power Scheduling for Sensor Networks”, IPSN 2004, Berkeley, Calif., April 2004.
Polastre, Hill and Culler, “Versatile Low Power Media Access for Wireless Sensor Networks”, Pedamacs (Coleri, UCB) SMAC (UCLA).
What is needed is a communication system that is especially adapted to communication environments with a variety of random origins and random receivers, which is efficient, secure, reliable, scalable, and low power.